The present invention relates to electrochemical cells and more particularly to electrochemical cells incorporating an improved lithium-containing anode.
Lithium and lithium alloys have been suggested as negative electrodes in electrochemical cells because lithium is highly electronegative and lithium and its alloys have low atomic weights. The combination of high electronegativity plus low atomic weight makes possible the construction of high energy density cells. Although lithium and lithium alloys have many desirable characteristics as use for anode materials, there exist problems that have limited their use as battery anode material.
Lithium is highly reactive and readily reacts with a number of potential organic solvents. Such reactions in a battery environment result in undesirable self-discharging and consequently solvents that react with lithium cannot be used to dissolve appropriate lithium salts to form the electrolyte. It has been suggested that this problem could be overcome by alloying the lithium with a less reactive metal such as aluminum. Where lithium has been alloyed with aluminum to overcome the foregoing problems, alloys containing less than 20 weight percent lithium and preferably in the range of 5 to 20% by weight lithium (or on an atomic basis 30 to 50% lithium) have been employed. The presence of such large amounts of aluminum lowers the reactivity of the lithium but it has the disadvantage of increasing the weight of the anode (aluminum is more than five times as dense as is lithium) and the anode is rendered about 0.3 volt more positive as compared to a pure lithium reference anode.
Another reason for using lithium as an anode material is that lithium is inherently rechargeable, i.e. it can be electrodeposited from lithium ion-containing organic solutions. However, lithium's practical rechargeability is poor because metallic lithium is electrodeposited in dendritic form which can eventually lead to shorting out of the cell. In order to minimize the effects of dendritic growth, it has been suggested to employ cell separators, such as permeable membranes, which act as physical barriers to dendritic growth. Although cell separators are initially effective, lithium dendrites can eventually penetrate the cell separators and establish transient or permanent electronic shorts. Such dendritic growth must be minimized if lithium or lithium-containing alloy anodes are to be widely used.